Science: Human Evolution Takes a Twist With Australopithecus sediba

A set of six research reports, accompanied by an introductory essay in the 12 April issue of Science serves up a complete view of the anatomy and likely habits of Australopithecus sediba, based on the 2-million-year-old fossil remains of several partial skeletons found in 2008 at the site of Malapa, near Johannesburg, South Africa.

The studies show that Au. sediba possessed a mosaic of human and ape-like traits, with a human-like pelvis, hands and teeth, and a chimpanzee-like foot. Researchers still aren’t sure where Au. sediba fits into the hominin family tree, but the Malapa fossils are a striking example of the evolutionary events that led to the rise of early humans.

Composite reconstruction of Au. sediba (middle) based on recovered material from MHI, MH2 and MH4, and the research presented in the accompanying manuscripts. For comparison, a female modern H. sapiens is shown on the left, and a male Pan troglodytes (chimpanzee) on the right. [Credit: Lee R. Berger. (Courtesy of Berger and the University of the Witwatersrand)]

These latest articles, following up on the group’s earlier research probe deeper into the anatomy of a younger male skeleton, commonly referred to as MH1, and a female skeleton, known as MH2, as well as an isolated adult tibia or shinbone, known as MH4.

Joel Irish at Liverpool John Moores University and colleagues examined the dental features of MH1 and MH2. Like the rest of Au. sediba, the teeth are a collage of primitive and human-like features. Like humans, Au. sediba had five cusps or groove patterns on its molar teeth. These and other dental features indicate that Au. sediba is closer to South African australopiths like Au. africanus, a group of hominins that lived several million years ago and are thought to be early members of the human lineage.

The analysis of Au. sediba’s lower limb anatomy by Jeremy DeSilva at Boston University and colleagues suggests that Au. sediba walked in an entirely unique way. Au. sediba‘s small heel resembled a chimp’s heel. The species likely walked with an inward rotation of the knee and hip, with its feet slightly twisted.

This primitive way of walking might have been a compromise between upright walking and tree climbing, the authors concluded, since Au. sediba seems to have had more tree-climbing adaptations in its anatomy than other australopiths. The findings suggest that different species of australopiths moved differently—some climbed trees, some walked on the ground, and some did both.

Darryl de Ruiter at Texas A&M University and colleagues examined the mandibular or lower jaw remains of MH2, revealing a jawbone that differed from other australopiths in both size and shape.

Steve Churchill at Duke University and colleagues analyzed the upper limbs of Au. sediba. Excluding the hands and wrist, Au. sediba‘s upper limbs were primitive, suggesting the retention of climbing habits and the ability to suspend from trees.

Peter Schmid at the University of Zürich and colleagues studied the remains of Au. sediba’s rib cage, uncovering a narrow, ape-like upper rib cage, different from the broad, cylindrical chest seen in humans. The less well-preserved lower rib cage fossils were more human-like.

Scott Williams at New York University and colleagues analyzed two partial vertebral columns of Au. sediba. The team found that Au. sediba possessed the same number of lumbar vertebrae as modern humans, but sported a longer and more flexible lower back. Au. sediba‘s flexible spine differed from earlier australopiths and was more similar to that of Homo erectus, a later member of the human lineage.

Collectively, these studies point to Au. sediba as a likely ancestor of Homo, the genus that includes modern humans. The findings hint at the possibility that South African hominins like Au. sediba and perhaps Au. africanus did not descend from East African species like Au. afarensis.

Further unearthing of fossils from the Malapa site in the coming years could bring more surprises.

The reconstructed skull and mandible (far left) of Australopithecus sediba. Right maxilary (near left) and right mandibular dentition (far right) of Au. sediba MH1. [Skull reconstructed by Peter Schmid. Credit: Lee R. Berger. (Courtesy of Berger and the University of the Witwatersrand)] [Teeth image courtesy of Science/AAAS]